I’m positive the narrator of Caillou is either

  1. adult-aged Rosie, now a comedian/novelist, romanticizing how great her brother was when they were kids, because Caillou has since died in a drug deal
  2. the grandmother who does NOT appear in the show, who has a restraining order against her, imagining what her grandchildren are like, since she is not allowed to see them

Related: I’m pretty sure “Sam” of “Stella and Sam”, is the now-30-something narrator, and Stella is now dead of something irresponsible when she was a rebellious teen.